The present invention relates to a shim between a tappet and a cam in a valve operating mechanism for an internal combustion engine, and in particular, a shim which increases oil-keeping capability on the surface which the cam slidably contacts, thereby decreasing wear.
FIG. 3 illustrates a conventional direct-acting type valve-operating mechanism used in a DOHC engine. A tappet body 1, which comprises a cylinder closed at the upper end, is slidably inserted in a bore 2a of a cylinder head 2. In a circular recess 3 on the upper surface of an upper wall la of the tappet body 1, a shim 4 for adjusting a valve space is detachably engaged. A cam 5 which has a nose 5a contacts the upper surface of the shim 4, the cam 5 having a center on extension of the axis of the tappet body 1. A wear resistant tip 6 is in a press fit with the upper wall 1a of the tappet body 1. A shaft of the engine valve 7 is provided in the tappet body 1, and the end of the shaft contacts the lower surface of the tip 6.
A spring retainer 9 is engaged with the upper end of an engine valve 7 via a pair of cotters 8 which surround the outer circumferential surface of the valve 7, and a valve spring 10 is provided between the spring retainer 9 and a base (not shown) of the cylinder head 2. The engine valve 7 is usually energized upwards by the valve spring 10, and closes a port (not shown) of the cylinder head 2.
The shim 4 is engaged with the cam 5 which rotates with rotation of an engine, and is periodically subjected to high load, so that it is susceptible to wear.
When worn, the shim 4 increases valve space, thereby generating large mechanical noise, or decreases the lifting range of the engine valve 7, thereby decreasing an opening area of the port to involve output power reduction. To overcome these disadvantages, the shim must be made of high-strength wear-resistant material, and it is necessary to decrease wear by increasing oil-keeping capability of the surface which contacts the cam 5. However, a conventional shim has a smooth flat upper surface, thereby decreasing oil-keeping capability. Thus, supplied lubricating oil is lost from the surface of the shim for relatively short time, thereby increasing frictional resistance and causing wear in the shim and the cam.